Apparatus for deviation borehole drilling

ABSTRACT

A bent-sub, which is a deviation control device, has arcuate stabilizing surfaces on opposite sides of the plane containing the direction of deviation of the borehole, the maximum width between the surfaces being nearly equal to but less than the diameter of the borehole and an arcuate knee contact area, so that when the direction of the deviation is changed by rotating the drill string, the bent-sub is encouraged to slide, rather than roll, around the borehole wall and greater control can be exercised over the direction of deviation.

United States Patent [72] Inventor Howard Trethewen Edgecombe 2,712,434 7/1955 Giles et al. 175/73 Filton, England 2,819,039 1/1958 Lindsay et al... 175/73 {21] Appl. No. 794,222 2,819,040 1/1958 James et al. 175/76 [22] Filed Jan. 27, 1969 3,306,378 2/1967 Williams 175/325 [45] Patented Aug. 3, 1971 Prim qty Examiner-James A. Leppmk [73] Awgn 32:33. Assistant Examiner-Richard E. Favpeau Priority Feb. 2 Attorney-Mawhmney & Mawhinney 3 3] Great Britain [31 I 05557/68 [54] APPARATUS FOR DEVIATION BOREHOLE DRILLING 7 Claim 6 D In F its ABSTRACT: A bent-sub, which is a deviation control device, US. cl-

hu atcuate stabilizing surfaces on opposite sides of the plane [51] 7/04 containing the direction of deviation of the borehole, the max- D' n between the surfaces being nearly equal to but 75, 399, 325, 398,408. 61 less than the diameter of the borehole and an arcuate knee contact area, so that when the direction of the deviation is [56] Cm I changed by rotating the drill string, the bent-sub is encouraged UNITED STATES PATENTS to slide, rather than roll, around the borehole wall and greater 2,336,333 12/1943 Zublin 175/75 X control can be exercised over the direction of deviation.

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I I l I I l Patented Aug. 3, 1971 K 3,596,721

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Patented Aug. 3, 1971 3,596,721

3 Sheets-Sheet 2 ll/Euro R Howmp Tlrsru: n/Ep/ Ema- COMBE APPARATUS FGR DEVIATION BOREI-IOLE DRILLING The present invention relates to deviation borehole drilling and has particular reference to apparatus for controlling the deviation heading in azimuth of a drilling bit, driven by a downhole motor, e.g. turbodrill.

In turbodrilling a plurality of hollow collars joined end to end in a borehole form the lower end of a drill string through the center of which a mud flush is pumped from the ground surface. The md flush passes through a turbine in the turbodrill itself to drive the drill bit, and then returns to the surface along a passage, usually annular, between the outside of the drill collars and the borehole wall.

In deviation drilling, a bent-sub, which comprises a hollows collar through the bore of which mud flush may be pumped, and having one angled end face to provide the directional angle, is inserted in the drill string just above the turbodrill to cant the drill to one side in the plane of the azimuth heading of the bit.

That part of the bentsub which faces away from the angled face, known as the knee, contacts the borehole wall over an area known as the knee contact area. The bent-sub as a fulcrum thus reacts on the borehole wall and a bending moment is exerted on the drill in the heading plane to cause a bit side load in the heading direction.

In drilling a small diameter borehole, for example one of 9 inches diameter, the bent-sub and the turbodrill are usually not much smaller than the drill bit, for example, 8 inches. However, as the required borehole size is increased the difference in diameter between the drill bit and the turbodrill and its bent-sub becomes greater. 7

When drilling a borehole, for example 12 inches diameter, the turbodrill diameter and its bent-sub diameter are both considerably less than the drill bit diameter, for example, they may be 8 inches diameter. In these circumstances with a bentsub angle of 1 to 3 it has been found that when the azimuth heading is changed by rotating the top end of the drill string, the knee contact area of the cylindrical bent-sub may roll round the wall of the borehole. Since the drill bit is not capable of lateral movement and the drill is free to rotate on its shaft, the rolling of the bent-sub round the borehole wall will cant the drill sideways in the desired heading plane but the drill bit will tend to head in the opposite direction. Similarly when the drill torque is varied, the bent-sub will tend to roll as the drill string winds or unwinds. Control of azimuth direction of the bit path thus becomes uncertain.

According to the present invention a bent-sub having end faces, one of which is angled to provide a directional angle for deviation drilling, comprises a body portion which has an arcuate surface forming at least a portion of the knee contact area of the bent-sub, and having further arcuate stabilizing surface portions for contacting the sides of a borehole, the surface portions being disposed on opposite sides of the plane containing the directional angle of the bent-sub, the maximum vwidth of the bent-sub perpendicular to said plane being substantially equal to but less than the diameter of the drill bit with which the bent-sub is to be used, means for connecting a drill to one of the end faces of the bent-sub, the means being positioned so that the center of the drill lies in said plane and is offset from the mass center of the body portion towards the knee contact area, the minimum distance of the drill center from the knee contact area being one drill radius.

A bent-sub incorporating these features will be encouraged to slide round a borehole wall by contact of the stabilizing surfaces on the wall, and this will be particularly true where the stabilizing surfaces and the arcuate knee contact area all lie on a continuous arc.

Preferably the bent'sub is made as large as possible in the transverse plane without choking the passage for the return flow of mud flush. By this means any lateral vibrations which may occur in the drill string above the bent-sub will be at least partially restricted before being transmitted to the drill.

One end portion of the bent-sub, which will be the top end in use, may, however, be cylindrical and may be of reduced radius in relation to the knee contact area in order to facilitate fishing of the bent-sub and drill from the borehole. Preferably said reduced end portion is formed eccentrically with the main body of the bent-sub and the center bore of the bent-sub is formed concentrically with said end portion.

The bent-sub may be made from cylindrical stock of diameter only slightly less than that of the borehole and may then have the side opposite the knee contact area reduced in dimension to form a passage for the return flow of mud flush.

Alternatively the bent-sub may be made from cylindrical stock of diameter substantially less than that of the borehole so that the passage for the return flow of mud flush is not substantially blocked, and subsequently have theknee contact area of larger radius of curvature formed thereon.

In a further alternative embodiment lengths of drill collar are attached to a conventional bent-sub to provide the stabilizing surfaces.

The invention will now be particularly described in more detail, merely by way of example, with reference to the accompanying drawings in which:

FIG. 1 illustrates an elevation of a bent-sub made according to the present invention.

FIG. 2 illustrates a plan view of the bent-sub of FIG. 1 in position in a borehole.

FIG. 3 illustrates a plan view of the bent-sub of the present invention made in an alternative manner.

FIG. 4 illustrates a further alternative construction of the bent-sub of the present invention.

FIG. 5 is an elevation of yet a further alternative construction of the invention, and,

FIG. 6 is a plan view of the bent-sub of FIG. 5.

Referring now to FIGS. 1 and 2 of the drawing there is shown a bent-sub suitable for use in a IPA-inch diameter borehole with an 8-inch diameter drill. The bent-sub has a body portion 1 which is of substantially cylindrical shape having an angled face 2 at one end. In this example the bent-sub is made from stock initially 3 feet long and of 10% inches diameter, the center of the stock being shown at A. A portion 3 of the surface of the body portion 1 of the bent-sub approximately l-foot long thus has a radius of curvature of 5% inches centered on A. An end portion 4 approximately 2-feet long and required for fishing, is reduced to 4 inches radius on center A and a 3-inch diameter bore 5 is also formed on center A through the whole length of the bent-sub. An area 6 is formed, which will provide the knee contact area, and which faces in the opposite direction to the angled end face. This area 6 is machined from a center B with a radius of curvature of 5%' inches, which is substantially equal to but less than the radius of the borehole 7 and the center B is offset from center A in the direction of the angled end face, so that the circles of cross section of surface 6 and the end portion 4 have a common tangent on the knee side of the bent-sub at C. Thus the mass center of the larger body portion 1 of the bent-sub is eccentric with respect to the center bore and the fishing length 4, and hence allows a greater cant at the top end of the drill. This enables a larger bent-sub angle to be used and gives greater directional control.

It will be understood that means, usually in the form of screw threads 8, must be provided for attaching the bent-sub to the drill collars and drill motor.

When the azimuth heading, i.e. the direction of deviation, is changed the bent-sub will try to roll round the borehole as before but, areas 6 of the surface of the body 1 will contact the side of the borehole and the bent-sub will then be encouraged to slide around the hole rather than to roll. Making the surface portions a continuation of the arc of knee contact area improves the skidding tendency, and further, the nearer the maximum width of the bent-sub on a line perpendicular to the plan containing the direction of deviation, is to the diameter of the borehole, of the drill bit, the greater will be the tendency to skid.

A bent-sub having the above-described features has a number of advantages:

1. By more nearly matching the curvature of the knee contact area to that of the wall of the borehole 5, the bentsub is encouraged to skid round the all of the borehole when the azimuth heading is changed. Thus the drill is always canted and bent in the plane of the heading direction and bit directional control is improved.

2. Since the knee contact area is larger than heretofore there is less localized wear despite the skidding, and changes of heading may be made smoothly with less likelihood of the bent-sub sticking against irregularities of the borehole wall.

3. By providing the bent-sub with a large eccentric mass,

vibrations emanating from the drill string above the bentsub are largely restricted, and the skidding tendency of the knee contact area provides friction damping at the top end of the drill. Those parts of the surface of the bent-sub at its widest section at right angles to the direction of deviation, being nearly equal to the borehole diameter, will contact the borehole wall under the influence of lateral vibrations and act to stabilize the bent-sub.

4. The amount of bending imposed on the drill during withdrawal from the borehole is reduced, since there is limited room for lateral movement of the bent-sub in the bending plane. 5. The bent-sub may be readily fished in spite of its size and eccentricity. 6. The bent-sub may be made inexpensively since there is little additional machining involved. The means for connecting the drill and drill pipe to the bent-sub are conventionally screw threads which may be formed internally of the bent-sub at its ends, or alternatively on pins which project from the end faces and are externally screw threaded.

In either case the axis of the screw thread is made colinear with the axis of the internal bore which is one drill radius from the knee contact area, so that the drill casing will have a common tangent to the knee contact area at the interface.

Referring now to FIGS. 3 and 4 there are shown alternative methods of making a bent-sub suitable for use in a l2V4-inch diameter borehole.

In the FIG. 3 a borehole outline is shown at 5.

The bent-sub body portion 1 is made from 1 l /-inch diameter stock, the knee contact area of the lower portion has a radius of curvature of 5% inches from the center A of the l l%-inches diameter stock. The opposite surface of the lower portion of the bent-sub has a radius of curvature of 5% inches from a center B which is displaced from center A towards the knee contact area by 1% inches. Thus a passage 11 is available for the return flow of mud flush up the borehole 5.

The turning centers for the cylindrical upper portion 12 of the bent-sub of 8 inches diameter and the 3 inches diameter bore 13 are also coincident at B. At both ends the bore 13 is opened out and internal screw threads (not shown) are formed therein.

FIG. 4 illustrates a further method of producing a bent-sub of the required configuration from a cylindrical stock of 11% inches diameter. In this case, in the lower part, the knee contact area 20 has a radius of curvature of 5% inches from the center A of the stock but the opposite surface is formed with two flats 21 formed on the stock to make a suitable passage area 22 for the return flow of mud flush.

The upper portion 23 of the bent-sub is turned down to 8 inches diameter on a center B offset by 1% inches towards the knee contact area from the center A, and the bore 24 is also an existing 8-inch diameter bent-sub with little machining and hence low cost.

In this construction, two lengths of drill collar 31 are attached to the bent-sub with lobes at approximately The collar lengths 31 provide the body portion of the bent-sub with stabilizing surface portions which are symmetrical about the plane of the bent-sub direction. The maximum width of the bent-sub perpendicular to said plane is thus the length of the line A-A in FIG. 6 and this is arranged to be one-inch less than the diameter of the borehole.

The drill itself is attached to the square face 32 of the bentsub by the usual internal thread in the mud bore, so that the drill center is offset towards the knee contact area 33 with respect to the mass center of the bent-sub as a whole, thus providing a cant at the top of the drill.

The collar lengths 31 may be welded on the bent-sub body portion or may be detachably fixed, as by bolts, so that they can be changed for different diameter length for use with different diameter drill bits.

In general it is preferable to attach the drill to the square end face of the bent-sub rather than the angled face, since the actual contact between the knee contact area and the borehole wall will then tend to be at the top of the bent-sub, the bottom of the bent-sub and hence the top of the drill will thus be held off the wall, and there will be no risk of damage to the drill. If the angled end face is attached to the drill, the drill must not project beyond the circumference of the arc of the knee contact area.

With the construction of the FIG. 5 embodiment, the stabilizing surfaces also project close to the borehole wall opposite to the knee contact area and hence vibrations in a plane at right angles to the line A-A are effectively damped.

I claim:

1. A bent-sub having end faces, one of which is angled to provide a directional angle for deviation drilling and comprising a body portion which has an arcuate surface defining a knee contact area of the bent-sub, and having further arcuate stabilizing surface portions, for contacting the sides of a borehole, the surface portions being disposed on opposite sides of the plane containing the directional angle of the bentsub, the maximum width of the bent-sub perpendicular to said plane being substantially equal to but less than the diameter of a drill bit with which the bent-sub is to be used, means for connecting a drill to one of the end faces of the bent-sub, the means being positioned so that the center of the drill lies in said plane and is offset from the mass center of the body portion toward the knee contact area, the minimum distance of the drill center from the knee contact area being one drill radius.

2. A bent-sub as claimed in claim 1 and in which means is provided for connecting a drill to the end face of the bent-sub which is opposite to the angled end face.

3. A bent-sub as claimed in claim 1 and in which the means for connecting the drill is a screw thread.

4. A bent-sub as claimed in claim 3 and in which the end of the bent-sub to which the drill is to be attached is formed with an internal screw thread.

5. A bent-sub as claimed in claim 3 and in which the axis of the screw thread is one drill radius from the knee contact area.

6. A bent-sub as claimed in claim 1 and in which the stabilizing surface portions and the arcuate surface form a continuous 

1. A bent-sub having end faces, one of which is angled to provide a directional angle for deviation drilling and comprising a body portion which has an arcuate surface defining a knee contact area of the bent-sub, and having further arcuate stabilizing surface portions, for contacting the sides of a borehole, the surface portions being disposed on opposite sides of the plane containing the directional angle of the bent-sub, the maximum width of the bent-sub perpendicular to said plane being substantially equal to but less than the diameter of a drill bit with which the bent-sub is to be used, means for connecting a drill to one of the end faces of the bent-sub, the means being positioned so that the center of the drill lies in said plane and is offset from the mass center of the body portion toward The knee contact area, the minimum distance of the drill center from the knee contact area being one drill radius.
 2. A bent-sub as claimed in claim 1 and in which means is provided for connecting a drill to the end face of the bent-sub which is opposite to the angled end face.
 3. A bent-sub as claimed in claim 1 and in which the means for connecting the drill is a screw thread.
 4. A bent-sub as claimed in claim 3 and in which the end of the bent-sub to which the drill is to be attached is formed with an internal screw thread.
 5. A bent-sub as claimed in claim 3 and in which the axis of the screw thread is one drill radius from the knee contact area.
 6. A bent-sub as claimed in claim 1 and in which the stabilizing surface portions and the arcuate surface form a continuous arc.
 7. A bent-sub as claimed in claim 6 and in which said continuous arc is cylindrical. 